1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus and more particularly to, e.g. a digital camera having enhanced pixel density, such as a mega-pixel camera, and capable of shooting a subject with a selectable photographic sensitivity.
2. Description of the Background Art
It is a common practice with a digital camera to allow the user of the camera to select desired one of a plurality of photographic sensitivities. Sensitivity or photographic sensitivity refers to ISO (International Organization for Standardization) sensitivity conventionally for use in the sensitivity of photographic film. More specifically, a digital camera is often configured to allow the user of the camera to select either one of a low-sensitivity mode and a high-sensitivity mode corresponding to the ISO sensitivity ranging from 100 to 400 and the ISO sensitivity of 800 and over, respectively.
When such a digital camera is used to shoot a desired subject, an exposure time is reflected by the amount of signal charges generated. More specifically, the amount of signal charges generated in pixels, or photosensitive cells of the camera, increases in the low-sensitivity mode due to a longer exposure time, or decreases in the high-sensitivity mode due to a shorter exposure time.
The signal charges thus stored in the photosensitive cells are read out onto vertical transfer paths implemented as (charge-coupled devices (CCDs) and then transferred to a horizontal transfer path along the vertical transfer paths. The signal charges, i.e. electrons stored in the photosensitive cells are lost as the time elapses due to, e.g. thermal diffusion, as known in the art. The amount of signal charges exponentially decreases with the elapse of time. As a result, even when the same amount of signal charges is generated between the photosensitive cells close to the horizontal transfer path and those far from the same, the amount of signal charges actually be transferred to the horizontal transfer path differs from each other, thus degrading the quality of a resulting image.
Further, while the signal charges stored are sequentially transferred via potential wells formed in the individual transfer CCDs, a certain amount of electrons or signal charge remains in each CCD although it is extremely small. Consequently, the residual signal charges in the transfer CCDs will be mixed with signal charges which will follow, when transferred to the same transfer CCDs, in the next signal transfer and be different in color from the residual signal charges. This mixture of different colors (referred to as color mixture hereinafter) is another cause of the deterioration of image quality.
The deterioration of signal charges which is brought about due to the above causes when the charges are transferred by one stage of transfer is generally referred to as transfer efficiency. Color mixture is therefore apt to easily occur when the signal charges are transferred via a number of transfer stages. The transfer efficiency is an issue relating not only to the vertical transfer paths but also to the horizontal transfer path of an imaging device.
A modern solid-state image pickup apparatus is required to have horizontal transfer efficiency high enough to match with the several million to ten and several million pixels and the increasing photographic sensitivity of the apparatus. To meet this requirement, there has been a proposal to increase a drive frequency assigned to the horizontal transfer path for thereby guaranteeing the frame rate regulated.
However, a solid-state image pickup apparatus, originally low in yield, has a problem that transfer efficiency on the horizontal transfer path is lowered by the same causes as stated in relation to the vertical transfer paths. Particularly, the latter cause stated above allows a certain amount of signal charges to be left in the individual CCDs generally without regard to the quantity of incident light. As a result, when an image is generated by a small amount of signal charges as in the high-sensitivity mode, the influence of color mixture becomes critical because of the intrinsically small amount of signal charges. The influence of color mixture becomes more critical as the number of transfer stages increases. More specifically, color shift or offset is conspicuous in a picture having a gray or white area.